In rail car brake systems which are controlled by the pressure in a brake pipe or supervisory pipe which extends through the train, it has long been the practice to configure the system to apply the brakes when this pipe is broken or otherwise depressurized due, say, to accidental uncoupling. When however it is desired to intentionally uncouple two adjacent cars, the system should also permit uncoupling without a depressurization of the pipe and its attendant brake application. Various schemes have been devised for performing these two functions; but usually they have been unnecessarily complex or unreliable in use.